Incandescent lamps (or bulbs, or light bulbs) operate differently than light emitting diodes (LEDs). (LEDs are also referred to as “LED lamps”.) Basically, incandescent lamps are voltage driven devices, and LEDs are current driven devices. They therefore require different techniques for controlling (e.g., dimming) the intensity of their respective light outputs.
For an incandescent lamp the amount of light produced is more-or-less proportional to the cube of the applied voltage—more specifically, it is proportional to the 3.4th power of the ratio of the RMS (Root-Mean-Square) voltages. (The RMS voltage is the square root of the mean value of the square of the instantaneous voltage.) That means, if the RMS voltage is reduced by one half (½), the light is reduced to (½)3.4 or about one-tenth ( 1/10). Or, an incandescen lamp operating at 20% of nominal voltage (e.g., a 5V lamp operated at 1V) will only produce 1/280 of the light produced at nominal voltage.
In an alternating current (AC) system, such as household, adjusting the light output of an incandescent lamp lighting device is typically carried out by controlling a phase angle of an electrical power supplied to the incandescent lamp under application of a switching element such as a TRIAC switch. In a direct current (DC) system, such as instrument illumination, adjusting the light output of an incandescent lamp is typically carried out by lowering the voltage, as discussed in the previous paragraph.
LEDs are current driven devices. Fewer amps in means fewer photons out. But LEDs are not typically powered from a true current source. The vast majority of power supplies are voltage sources, and that is what is usually found powering LEDs. The relationship between current and voltage in an LED is non-linear. As the voltage increases from zero there is only a trickle of current and no noticeable light. At about a volt and three-quarters (e.g., for red LEDs) the current begins to increase appreciably and the first glimmer of light appears. At two volts (2V) the LED is bright and with only a little more voltage it becomes very bright. Once over about 2.2 volts, the current rapidly soars beyond safe operation. Generally speaking, powering LEDs directly from a voltage source is not such a good idea. The usual approach is to connect a resistor in series with the LED. The combination is still non-linear, but in a much more well behaved manner. In fact, over the range of safe operating current, it acts incrementally linear. And, of course, series resistors represent power losses.
It takes 2 to 4 V to light an LED, depending on its color. Red LEDs take about 2 V, green and yellow LEDs take about 2.4 V, blue LEDs take 3 V, and white LEDs take about 4 V to illuminate. For white LEDs operating normally, a voltage of at least approximately 3.5 volts is required. These voltages are referred to as “turn-on” (or “threshold”) voltages. For a given type of device, run in different batches, the threshold voltages may vary by several percent (e.g., +/−10%) from rated threshold.
A typical technique for dimming LEDs is to use Pulse Width Modulation (PWM). Pulse Width Modulation is a technique of voltage regulation. In PWM, the output is at an essentially fixed voltage level, and it is turned on-and-off at a high rate. In this manner, an average output voltage is equal to the duty cycle “D” (the percentage of time that the output is on) times the input voltage. A typical frequency of the output on-and-off would be 400 Hz. The duty cycle D can be varied from 0% (resulting in 0 volts average output voltage) all the way to 100% (average output voltage=input voltage). The light produced by LEDs is proportional to the average current in the LEDs. Applying a 50% duty cycle PWM signal to an LED will produce ½ of the light that the LED produces when 100% duty cycle is applied.
Many existing integrated circuits perform LED dimming. These operate in either PWM mode if the input voltage is higher than the LED voltage, or they operate in current-source mode if the input voltage is lower (like battery operated equipment). Some current source devices can be PWM'ed to vary intensity as well. Typical examples of current source devices are the CAT32 CMOS White LED Driver (Catalyst Semiconductor Inc.) and the FAN5608 Series/Parallel LED Driver with Current-Regulated, Step-Up DC/DC Converter (Fairchild Semiconductor).
Like many other display systems, aircraft instrumentation displays frequently employ illuminated indicators. Originally incandescent bulbs were employed for this purpose. However, a variety of factors have motivated replacement of incandescent bulbs with light emitting diodes (LEDs) in such applications, including improvements in power consumption, heat generation, vibration resistance, and operating lifetime.
This leads to situations where on a given aircraft instrument panel, there is a mixture of incandescent lamps and LEDs. Since techniques for controlling illumination are different for each, this creates some challenges, and in “next generation” cockpits (aircraft designed less than 10 years ago) there may be two cockpit lamp power busses, one for incandescent lamps, the other for LED lamps, each with its own system for dimming.
One approach to satisfying the luminance standards and voltage level expectations when utilizing LEDs involves providing a mechanism for compensating for changing the portion of the applied input power which is actually transmitted to the LEDs. The portion of the applied input power which is transmitted to the LEDs changes across the operating range of input power to the LED illuminated indicator, matching the power transmitted to the LEDs to the power which is required by the LEDs to achieve approximately the same luminance as an incandescent bulb receiving the same input power. However, this approach negates at least some of the reduction in power consumption achieved by employing LEDs in lieu of incandescent bulbs.
Another approach to forcing multiple LEDs to “behave” like incandescent lamps (bulbs) for purposes of dimming (e.g, to follow the incandescent voltage/intensity curve), so that the LEDs could then be powered by an existing incandescent dimmer (DC-mode dimmer) and achieve brightness tracking, can be found in U.S. Pat. No. 6,670,776 which discloses enhanced trim resolution voltage-controlled dimming LED driver. Illumination sources, each including at least one light emitting diode, are connected either in series or in parallel by a switching circuit, depending upon an applied input voltage. The switching circuit switches the illumination sources from series- to parallel-connection, or vice versa, when the applied input voltage crosses a threshold value in traversing the operating range of applied input voltages. Because the light emitting diodes within the illumination sources are switched from series to parallel connection at a defined kickover point, the voltage-luminance characteristic changes on opposite sides of the kickover point. The resulting overall voltage-luminance characteristic has greater variability in luminance across the entire operating range of applied input voltages, and luminance-variance is not limited to only a portion of the operating range. Greater trim resolution for voltage-controlled dimming of the light emitting diode is therefore provided, with industry standard luminances being achieved at appropriate applied input voltages. This arrangement requires that the LEDs can be clustered to track the brightness curve of the equivalent incandescent lamps. These LEDs could then be powered by the existing incandescent dimmer (DC-mode dimmer) and achieve brightness tracking.
U.S. Pat. No. 5,430,356 discloses programmable lighting control system with normalized dimming for different light sources. A lighting control system is adapted to dim a plurality of groups of light sources in a room to any one of a number of different preset levels to achieve a like number of different lighting scenes. Each group of light sources defines a lighting zone of the same type of light source, for example, incandescent lamps, fluorescent lamps, neon lights, etc. The system includes a plurality of dimmers for adjusting the respective light levels of the different lighting zones, and a display panel for displaying the instantaneous light level of each zone. A suitably programmed microprocessor or the like operates to normalize the system's dimming performance for a plurality of different types of light sources so that a given change in dimmer setting produces the same change in perceived light level from each of the different types of light sources. Preferably, a system user inputs the type of light source used in each zone by a software scheme that operates the light level indicators of the display panel in an alternative mode to indicate the various types of light sources. In this patent, the intensity tracks among different kinds of lights, but each kind of light gets its own dimmer.
U.S. Pat. No. 6,559,777 discloses a lighting system wherein an infrared (LED) and an incandescent lamp cluster are powered by the same power buss. In this system, different voltage levels are transmitted to the infrared/incandescent lamp cluster. A power converter in the lamp cluster detects the voltage level and converts the power to an appropriate voltage level sends the power to one or the other of the infrared and incandescent lamps. This system has the added complexity and decreased reliability of power conversion electronics in each lamp. This function of this system is not to match the intensity of the incandescent lamp with the LED lamp; each type lamp is operated exclusively of the other. The system also requires special composite lamps.